1. Technical Field
The present invention relates to a polarization conversion element, a projector using the polarization conversion element, and a method for manufacturing the polarization conversion element.
2. Related Art
There is a known projector of related art in which a light modulator modulates light outputted from a light source apparatus in accordance with image information to form an optical image and the formed optical image is projected. A projector of this type includes a polarization conversion element that is placed in a position between the light source apparatus and the light modulator and aligns polarization axes of light rays that form the light outputted from the light source apparatus with one another to increase light usage efficiency. In recent years, a trend toward high-luminance projector requires improvement in light fastness and other properties of the polarization conversion element.
JP-A-2011-118257 discloses a configuration in which a first light transmissive member, a second light transmissive member, and a plasma polymerization film that allows molecular junction between the first and second light transmissive members are stacked on each other to form a polarization conversion element.
To apply the configuration disclosed in JP-A-2011-118257 to a polarization conversion element, characteristics of plasma-polymerization-film-based bonding require that the light transmissive members (light transmissive substrates) be thin, the number of stacked members be small, the light transmissive substrates have high surface precision, and other requirements be satisfied. The configuration described in JP-A-2011-118257 is only applicable to a compact polarization conversion element that satisfies the conditions described above.
To bond light transmissive substrates having thickness variation, warpage, surface undulation, and other disadvantageous properties to each other via a plasma polymerization film, it is necessary to use thin light transmissive substrates and sufficiently pressurize the light transmissive substrates to deform them before bonding. The light transmissive substrates can, however, be bonded to each other only when the bonding strength is greater than a restoring force of the light transmissive substrates that have been elastically deformed, whereas the light transmissive substrates cannot be bonded to each other and the bonded substrates separate from each other or otherwise deteriorate when the restoring force of the light transmissive substrates is greater than the bonding strength.
Further, since a bonding layer formed of a plasma polymerization film has an extremely small thickness ranging from about 0.2 to 0.5 μm, the bonding layer cannot be used as a buffer that buffers variation in thicknesses of the light transmissive substrates, warpage thereof, accumulated rigidity of the multi-layered light transmissive substrates, and other disadvantageous properties. Therefore, when a polarization conversion element is manufactured, the plasma polymerization film cannot buffer variation in size and form and other disadvantageous factors of the members that form the polarization conversion element, resulting in a manufacturing problem.
Further, in manufacturing a polarization conversion element, to bond a polarization separation film formed on one light transmissive substrate to another light transmissive substrate, a UV curable adhesive is typically used. A bonding layer formed of the UV curable adhesive has a thickness ranging from about 3 to 10 μm and can buffer variation in size and form and other disadvantageous properties of the members that form the polarization conversion element. The UV-curable-adhesive-based bonding is, however, undesirably inferior to the plasma-polymerization-film-based bonding in terms of light fastness and heat resistance.
It has therefore been desired to develop a polarization conversion element that has improved light fastness and can be readily manufactured, a projector using the polarization conversion element, and a method for manufacturing the polarization conversion element.